A ray of light is incident normally on a plan mirror. If the incident ray is kept fixed while the mirror is rotated through an angle of 30o, determine the initial and final angles of reflection respectively
0o and 60o
90o and 30o
0o and 30o
90o and 60o
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Discussions (14)
The right option is C.
For a plane mirror: If the incident ray falls along the normal, the angle of incidence is 0 degrees, not 90 degrees. The angle is measured with the normal to the mirror and not with the plane of the mirror. Hence the angle of reflection is equal to 0 degrees. So it retraces the path.
Answer is option A.
When the ray is incident normally, the angle is 0 degree not 90 degree.
For a plane mirror: If the incident ray falls along the normal, the angle of incidence is 0 degrees, not 90 degrees. The angle is measured with the normal to the mirror and not with the plane of the mirror. Hence the angle of reflection is equal to 0 degrees. So it retraces the path.
The answer is C, read the question properly and understand what you were asked to find.
People saying option A are actually calculating the angle between the final reflected ray and the initial reflected ray (incident/old normal ray in this case) and that's not what you were asked to find.
Now lets break it down; you were asked to calculate the initial and final angle of reflection respectively (the angle made to the normal by the incident ray and the angle made to the new normal by the new reflected ray respectively).....
1. Initial Condition (Before Rotation):
The ray is incident normally (perpendicular to the mirror).
This means the angle of incidence (i) = 0°, so the angle of reflection (r) = 0° as well.
2. After Rotating the Mirror by 30°:
When the mirror rotates, the normal to the mirror also rotates by 30°.
The incident ray remains fixed, so its new angle with the normal becomes 30°.
By the law of reflection, the angle of reflection must be equal to this new angle of incidence (30°).
Now, here's the key point:
The reflected ray rotates by twice the mirror’s rotation (2 × 30° = 60°) relative to its original direction, but the new angle of reflection is measured from the new normal, which is 30°.
Answer Choice Analysis:
A (0° and 60°) → Incorrect, because 60° is measured from the old normal, not the new one.
B (90° and 30°) → Incorrect, because the initial angle of reflection was 0°, not 90°.
C (0° and 30°) → Correct!
D (90° and 60°) → Incorrect, because the initial angle of reflection was 0°, not 90°.
Correct Answer: C (0° and 30°)
Your intuition about the new normal was spot on!
